There are numerous reasons for the liquefaction of gases, including naturally occurring gases such as methane. Perhaps the chief reason is that liquefaction greatly reduces the volume of a gas, making it feasible to store and transport the liquefied gas in containers of improved economy and design. Liquid gases can be stored in suitably designed cryogenic containers and dispensed into vehicle tanks using techniques that have been in use for many years in the industrial cryogenic gas industries.
Many industrial gases such as propane, butane and carbon dioxide can be liquefied by placing them under very high pressure. However, producing liquid from methane may not be achieved with high pressure alone. To this extent, methane, a cryogenic gas, is different from other industrial gases. To liquefy methane it is typically necessary to reduce the temperature of the gaseous phase to below about −160° C., depending upon the pressure at which the process is operated.
Numerous systems exist in the prior art for the production of liquefied natural gas (“LNG”). Conventional processes known in the art require substantial refrigeration to reduce the gas to liquid and maintain it at its liquefaction temperature. Among the most common of these refrigeration processes are: (1) the cascade process; (2) the single mixed refrigerant process; and (3) the propane pre-cooled mixed refrigerant process.
The cascade process produces liquefied gases by employing several closed-loop cooling circuits, each utilizing a single pure refrigerant and collectively configured in order of progressively lower temperatures. The first cooling circuit commonly utilizes propane or propylene as the refrigerant; the second circuit may utilize ethane or ethylene, while the third circuit generally utilizes methane as the refrigerant.
The single mixed refrigerant process produces LNG by employing a single closed-loop cooling circuit utilizing a multi-component refrigerant consisting of components such as nitrogen, methane, ethane, propane, butanes and pentanes. The mixed refrigerant undergoes the steps of condensation, expansion and recompression to reduce the temperature of natural gas by employing a unitary collection of heat exchangers known as a “cold box.”
The propane pre-cooled mixed refrigerant process produces LNG by employing an initial series of propane-cooled heat exchangers in addition to a single closed-loop cooling circuit, which utilizes a multi-component refrigerant consisting of components such as nitrogen, methane, ethane and propane. Natural gas initially passes through one or more propane-cooled heat exchangers, proceeds to a main exchanger cooled by the multi-component refrigerant, and is thereafter expanded to produce LNG.
Most liquefaction plants utilize one of these gas liquefaction processes. Unfortunately, the cost and maintenance of such plants is expensive because of the cost of constructing, operating and maintaining one or more external, single or mixed refrigerant, closed-loop cooling circuits. Such circuits typically require the use and storage of multiple highly explosive refrigerants that can present safety concerns. Refrigerants such as propane, ethylene and propylene are explosive, while propane and propylene, in particular, are heavier than air, further complicating dispersion of these gases in the event of a leak or other equipment failure. It would therefore be beneficial to eliminate the external refrigeration circuit(s) in a liquefaction plant.
One of the distinguishing features of a conventional liquefaction plant in the prior art is the large capital investment required. The equipment used to liquefy cryogenic gases in high volumes is large, complex and very expensive. The plant is typically made up of several basic systems, including a gas treatment system (to remove impurities from the initial feed stream), and liquefaction, refrigeration, power, storage and loading facilities. Materials required in conventional liquefaction plants also contribute greatly to the plants' cost. Containers, long runs of piping, and multiple-level tiers of other equipment are principally constructed from aluminum, stainless steel or high nickel content steel to provide the necessary strength and fracture toughness at low temperatures. It would therefore be beneficial to decrease the initial amount of capital investment needed to form a liquefaction plant.
Another distinguishing feature of a conventional liquefaction plant in the prior art is that as a result of its complexity and size, the plant, by necessity, is typically a fixed installation that can not be easily relocated. Even if a conventional plant can be physically relocated, such a move is very costly and requires the plant to be out of service for many months while plant systems, components and structures are disassembled, moved and then reassembled on a newly prepared site. It would therefore be beneficial to provide a liquefaction plant that is small and simple in design so that it can be easily relocated without significant operational down time.
There exists a multitude of current prior art methods for the liquefaction of natural gas. For example, U.S. Pat. No. 5,755,114 to Foglietta discloses a hybrid liquefaction cycle for the production of LNG. The Foglietta process passes a pressurized natural gas feed stream into heat exchange contact with a closed-loop propane or propylene refrigeration cycle prior to directing the natural gas feed stream through a turboexpander cycle to provide auxiliary refrigeration. The Foglietta process requires at least one external closed-loop refrigeration cycle comprising propane or propylene, both of which are explosive.
The system of U.S. Pat. No. 6,085,545 to Johnston first compresses the natural gas feed (typically methane) which then passes through an after-cooler to remove the heat of compression. At this point the natural gas flow is split into two flow portions, the first of which is cooled in at least one heat exchanger and then throttled into a collector, and the second of which enters a turboexpander wherein the temperature and pressure are lowered and the work of expansion is extracted. The second flow portion is then used in at least one heat exchanger as the heat exchange cooling medium.
U.S. Pat. No. 3,616,652 to Engel discloses a process for producing LNG in a single stage by compressing a natural gas feed stream, cooling the compressed natural gas feed stream to form a liquefied stream, dramatically expanding the liquefied stream to an intermediate-pressure liquid, and then flashing and separating the intermediate-pressure liquid in a single separation step to produce LNG and a low-pressure flash gas. The low-pressure flash gas is recirculated, substantially compressed and reintroduced into the intermediate pressure liquid. While the Engel process produces LNG without the use of external refrigerants, the process yields a small volume of LNG compared to the amount of work required for its production, thus limiting the economic viability of the process.
While these prior art inventions may be sufficient for the particular problems that they solve, it would be beneficial in the industry to provide an improved process for the cryogenic refrigeration and liquefaction of gases. It would also be beneficial to eliminate the external refrigeration circuit(s) in a liquefaction plant. It would be likewise be advantageous to decrease the initial amount of capital investment needed to form a liquefaction plant. It would also be advantageous to provide a liquefaction plant that is small and simple in design so that it can be easily relocated without significant operational down time.